In hunting down a metallic clanking sound I found that it was coming from my hub. If I grip the wheel at the top and push it from side to side, I can see that there is about a 2mm gap either between the cassette and hub or the middle hub and non-drive side part of the hub (depending of which way I am pushing). This is the source of my sound. It remains however tightly I set the rear skewer.
I'm thinking that there is either some way of tightening down the hub, or that the bike should have come with a spacer to account for the gap. Hub is Velomax and cassette is Campy Record.